1. Field of the Invention
This invention relates to methods for acquiring a satellite signal and satellite signal receiving devices.
2. Description of the Related Art
The Global Positioning System (GPS) is widely known as a satellite positioning system using a GPS satellite signal as a positioning signal, and is utilized in GPS receiving devices for mobile telephones, car navigation systems, or the like. GPS receiving devices generally perform a positioning calculation by calculating a three-dimensional coordinate and a clock error based on locations of GPS satellites and information such as pseudoranges between the GPS receiving devices themselves and GPS satellites.
A GPS satellite signal is a kind of communication signals that are spread-modulated in Code Division Multiple Access (CDMA) method, which has been known as a spread spectrum modulation method. GPS satellite signals are transmitted from GPS satellites on a frequency (carrier frequency) of 1.57542 GHz, which may not coincide with a receiving frequency at a GPS receiving device due to influencing factors such as a Doppler caused by movements of GPS satellites or the GPS receiving device. Hence, the GPS receiving device searches for a frequency to acquire a GPS satellite signal among receiving signals to determine an acquisition frequency.
Disclosed in JP A-2007-256111 is a general method for determining the acquisition frequency, by correlating a received signal and a replica signal of the spread-modulated signal from a GPS satellite while changing frequency and code phase. The method is also referred to as a correlation calculation in the frequency direction and code phase direction, or a frequency search and a code-phase search.
In a correlation calculation in the frequency direction, GPS satellites are moving fast and Doppler effects associated with the movement of the respective satellites can be significant. Such effect of the Doppler frequency may cause large error in the receiving frequency of a GPS satellite signal at the GPS receiving device. Another frequency error may also be caused by a clock error present in a local clock in the GPS receiving device. The GPS receiving device minutely changes frequency to correlate between the received signal and its replica signal within a frequency range accounting for an expected frequency error range. A lot of time is spent in the frequency search, hence the time it takes to acquire a GPS satellite signal is inevitably made longer.